There has been a continuing trend in the semiconductor industry toward smaller semiconductor devices with higher transistor density and an increasing number of input/output connections. This trend has led to semiconductor devices having an increased density of chip input/output connections and shrinking bond pad sizes. Semiconductor devices having small bond pad center to center distances are called fine pitch semiconductor devices. Wire bonding technology is currently being challenged by requirements of semiconductor devices having bonding pad center to center distances of less than 100 micrometers.
In semiconductor fabrication, wire bonding remains the dominant chip interconnection technology for fine pitch semiconductor devices. Gold or aluminum wire is commonly used to connect a bonding pad of a semiconductor die to a lead of the semiconductor device. Typically, ball bonding is used to connect the wire to the bond pad while wedge bonding, also called stitch bonding, is used to connect the wire to the lead. Commonly, a wire bonding apparatus including a capillary is used for both the ball bonding and the wedge bonding.
FIG. 1A shows a vertical cross section of a prior art capillary 111. The capillary 111 has a longitudinally extending wire feed bore 102 formed therethrough. The bore 102 typically includes a chamfer 105 that splays slightly outward towards the distal tip of the capillary 111 to an outer chamfer diameter 106. In operation, wire is fed downward through the wire feed bore 102, and out a bottom aperture 104 of the capillary 111.
FIG. 1B shows a vertical cross section of a prior art capillary 100 having a double chamfer structure. The capillary 100 has a first chamfer diameter 107 and a second chamfer diameter 109.
FIG. 1C shows a vertical cross section of the capillary 100 horizontally restraining a wire 110 while an electronic flame off mechanism (EFO) 112 applies energy to a distal end of the wire 110. The application of energy by the EFO 112 creates a free air ball 114 at the distal end of the wire 110. The wire 110 is held by a clamp (not shown) during this free air ball formation process. Size parameters of the free air ball 114 include a free air ball diameter 115. For a wire bonding apparatus using the capillary 100, the size of the free air ball 114 can be controlled by varying hardware and software parameters of the wire bonding apparatus. After formation of the free air ball 114, the clamp releases the wire 110 and the capillary 100 is used to bond the distal end of the wire 110 to a bond pad surface as explained below.
FIG. 1D shows a vertical cross section of the capillary 100 being used to form a ball bond 115 between the distal end of the wire 110 and a surface of a bond pad 116. The bond pad 116 is located on a semiconductor die which has a center to center bond pad distance (also called the bond pad pitch of the semiconductor device). After the formation of the free air ball 114, as explained above, the free air ball 114 (FIG. 1B) is forced downward to the bond pad 116 by the capillary 100. The capillary 100 is used in conjunction with thermal and ultrasonic energy to create the ball bond 115 between the distal end of the wire 110 and the bond pad 116. An anchoring area 103 represents the surface of the capillary 100 in contact with the ball bond 115. Size parameters associated with the ball bond 115 include a ball bond height 120, a standoff distance 123 and a footprint 122.
As the center to center bond pad distance (or bond pad pitch) is decreased in a semiconductor device, the size of the bond pad 116 is typically decreased. For example, a semiconductor device having a 70 micron bond pad pitch can have a 60 micron.times.60 micron bond pad 116. It is very difficult to consistently achieve a ball bond 115 small enough to fit on a bond pad 116 of this size using the capillary 100. The footprint 122 must be limited in order to prevent flash of wire metal over to an adjacent bond pad 116 thereby creating a short between adjacent bond pads 116. A short between adjacent bond pads 116 can result in operational failure of the semiconductor device.
With reference still to FIG. 1C, one problem with use of the capillary 100 is that it is difficult to precisely control the size of the ball bond 115. For a wire bonding apparatus using the capillary 100, the size of the ball bond 115 (including the ball bond height 120 and footprint 122) is dependent on the size of the free air ball 114 (FIG. 1B). Hardware and software parameters of the bonding apparatus must be adjusted to vary the size of the free air ball 114 (FIG. 1B). For a wire bonding apparatus using the capillary 100, the size of the ball bond 115 is also dependent on parameters such as the bonding power, capillary tip position, bonding force, component temperatures and the nature of the ultrasonic energy delivered during the formation of the ball bond 115. For ball bonding of fine pitch semiconductor devices, the tip dimension 108 of the capillary 100 can be reduced so that the capillary 100 can form a ball bond 115 small enough to fit on the small bond pad 116. However, reducing the outer diameter tip dimension 108 weakens the capillary 100 which is subjected to great stress particularly during wedge bonding as explained below. The most significant factors that decide the shape and strength of the ball bond 115 are the tip dimension 108 and second chamfer diameter 109 of the capillary 100.
Conventional ball bonding techniques require a combination of ultrasonic energy provided by an ultrasonic transducer coupled to the capillary 100 and thermal energy stored in the bond pad. Typically, the bond pad and semiconductor die must be pre-heated to at least 150 degrees Celsius which may have detrimental effects on the semiconductor die. As an example, for silicon used in most conventional wafer technology surfaces, preheating the bonding surface prior to ball bonding may result in degradation of dopant regions of the silicon substrate. Current ball bonding techniques also require ultrasonic energy input at no less than 60 mW. Ultrasonic bond power is kept to a minimum for a number of reasons including shorter tool longevity and reduced capillary tip position control associated with higher bond power levels. Bond force is also kept to a minimum for the same reasons as bond power. Lower force also results in less gold contamination at capillary face dimensions that ensure reliable wedge bonding.
FIG. 1E shows a vertical cross section of a wedge bond 124 formed by the capillary 100. The wedge bond 124 is formed between an extended length of the wire 110 and a surface of an inner lead 126 of a lead frame. Reducing the tip dimension 108 of the capillary 100, to reduce the size of the ball bond 115 as described above, causes degradation in strength of the wedge bond 124. This is due to the fact that the area in which the wedge bond 124 is formed depends on the outer diameter tip dimension 108 of the capillary 100. Therefore, a difficult problem with using the capillary 100 concerns the tradeoff between a small outer diameter tip dimension 108 for achieving small ball bonds and a larger outer diameter tip dimension 108 for achieving strong wedge bonds. In view of the foregoing, it should be apparent that improved wire bonding techniques would be desirable.